Rotor system having alternating length rotor blades and positioning means therefor for reducing blade-vortex interaction (BVI) noise

ABSTRACT

A Variable Diameter Rotor (VDR) system (4) having telescoping odd and even blade assemblies (O b , E b ) wherein the odd blade assemblies define a radial length R O  and the even blade assemblies define a radial length R E . Each blade assembly (O b , E b ) defines an internal chamber (64) for accepting a positioning means (70) operative to effect telescopic translation of the blade assemblies (O b , E b ) such that the radial length R E  of the even blade assemblies (E b ) is equal to the radial length R O  of the odd blade assemblies (O b ) in a first operating mode, and such that the radial length R E  is between about 70% to about 95% of the length R O  in a second operating mode. The positioning means (70) includes a centrifugal restraint assembly (80) disposed within each internal chamber (64) of the rotor blade assemblies (O b , E b ), a stop surface (64s) formed internally of each internal chamber (64), and an actuation means (90) operative for transpositioning the centrifugal restraint assemblies (80) within the internal chambers (64) such that, in one operating mode, the centrifugal restraint assemblies (80) are disposed in abutting engagement with the stop surfaces (64s) and, in another operating mode, the actuation means (90) is disposed in abutting engagement with the centrifugal restraint assemblies (80) of at least one of the blade assemblies (O b , E b ).

TECHNICAL FIELD

The present invention is directed to means for reducing the noiseradiated from the rotor blades of rotorcraft, and, more particularly, tomeans for reducing Blade-Vortex Interaction (BVI) noise radiated fromsuch rotor blades when operating in flight modes which produce highlevels of BVI noise.

BACKGROUND OF THE INVENTION

One concern of rotorcraft designers is to reduce, to the extentpracticable, the noise radiating from the rotor blades during flightoperations. In particular, landing approaches which are characterized bya low speed, descending flight profile produce significant noise levelsdue to acoustic emissions known as Blade-Vortex Interaction (BVI).Insofar as such flight profile typically occurs at low altitude and overpopulated areas, BVI noise presents a primary technical issue which mustbe resolved to obtain community acceptance, and, more importantly,certification of newly-developed rotorcraft.

During typical rotorcraft flight operations, the rotor blades create ahigh velocity, low pressure field over the upper aerodynamic surfacethereof and a low velocity, high pressure field over the loweraerodynamic surface. The pressure differential generates the necessarylift forces for flight operations but, additionally, effects thegeneration of vortices at the tips of the rotor blades. Morespecifically, the pressure differential engenders airflow circulationfrom the high pressure field to the low pressure field to create a tipvortex. A tip vortex is shed from one rotor blade and impinges/interactswith a subsequent rotor blade as it rotates through the vortex field.The interaction of a tip vortex with a rotor blade induces impulsiveairloading which creates an acoustic pressure wave that is the source ofBVI noise.

BVI noise is generally not a concern in ascending or cruise flight modesinasmuch as the rotor disk, i.e., the plane defined by the rotor blades,moves away from the vortex wake. Consequently, the vortices are distallyspaced from the rotor and do not significantly interact therewith. BVInteractions are most prevalent, however, during descent modes ofoperation, insofar as the downward velocity of the rotorcraft causes therotor to fly into its wake thus interacting with multiple vortices.

The trajectory and core strength of vortices are difficult to predict;however, it may be generally stated that the vortices move downward in aspiral pattern as a function of the speed and flight attitude of therotorcraft, the boundary conditions imposed by the fuselage, theturbulence of the atmosphere, and other factors such as the lift-timehistory of each rotor blade. Generally, BV Interactions which occur inthe first quadrant of the rotor disk (0 degrees being positionally aftof the rotor shaft axis and along the longitudinal axis of therotorcraft) generate strong BVI impulses due to the combined rotationalvelocity of the rotor blades and the forward flight velocity of therotorcraft. Furthermore, the probability for strong interactions isintensified due to the high concentration and strength of vortices inthis quadrant.

Another factor which influences the strength of the BV Interactionsincludes the orientation of the rotating vortex with respect to theimpinging rotor blade. The orientation of the vortex is defined by theangle of intersection between the leading edge of the rotor blade andthe centerline of the vortex, i.e., vortex core. When the orientation issubstantially parallel with respect to the rotor blade leading edge, thecirculatory flow of the vortex affects a large portion of the bladelength, and, consequently, excites large impulsive pressure waves.Orientations which are substantially perpendicular or oblique to theleading edge produce relatively benign interactions by limiting theblade spanwise extent that is exposed to the circulatory flow.

Yet another factor which determines the strength of BVI encounters isthe spatial separation between the rotating vortex and apassing/intersecting rotor blade. The spatial separation may be moreaccurately defined as the distance from the vortex core to a point onthe leading edge of the rotor blade. Insofar as the airflow velocity ata point in the vortex field is a function of 1/R, wherein R is thedistance from the vortex core, it will be apparent that the airflowvelocity, in theory, becomes infinite (∞) as R approaches zero (0) anddiminishes at a precipitous rate as the distance R increases.Accordingly, when the spatial separation is small, e.g., less than thethickness of the rotor blade, the rotating vortex field willsignificantly impact the circulation about the rotor blade therebyproducing large BVI impulses. Conversely, when the separation is larger,e.g., 5X the thickness of the rotor blade, the BVI impulse issubstantially reduced due to the precipitous decline of vortex fieldvelocity at the point of rotor blade intersection.

The rotorcraft designer, therefore, attempts, to the extent practicabletaking into account, inter alia, weight, cost, performance, and systemcomplexity, to incorporate elements into the rotor assembly thatmitigate the BVI noise radiated therefrom. There are several differentdesign options to mitigate BVI radiated noise. These approaches may begrouped into three broad categories, namely, passive systems, deployablepassive systems and active systems.

Passive systems attempt to reduce BVI noise by favorably altering rotorblade geometry or rotor operating parameters. Examples of passivesystems include selective tip shaping to diffuse or weaken the vortex.One design option involves a forward swept tip wherein the vortex isgenerated inboard of the tip, such inboard generated vortex being morediffuse, i.e., reduced in strength, than the tip vortex generated by aconventional rectangular tip. Another design configuration is a sub-wingtip wherein a sub-wing is attached to the rotor blade at the tip thereofto produce two weak, corotating vortices that mix downstream and diffusedue to viscous effects. Yet another design approach involves reducingtip speed, below about 675 ft/sec (206 m/sec), or increasing the numberof blades to reduce blade loading, and, consequently, the strength ofthe tip vortex. These design options provide marginal improvement inmitigating BVI noise, e.g., on the order of about 2 to 5 dBa reductionand, furthermore, often degrade the overall operating efficiency of therotor system. Furthermore, such design options may be difficult and/orcostly to implement.

Deployable passive systems alter the rotor blade geometry in flight bydeploying a noise reduction device during modes of operation whichproduce high levels of BVI noise. Examples of deployable passive systemsinclude half-plow vortex generators which are disposed along the upperor lower surface of a rotor blade and are deployable when the rotorcraftis in a descending flight profile. Similar to the sub-wing tip discussedabove, the half-plow vortex generators produce two or more vortices ofreduced strength in an attempt to disrupt the formation of a single,,more potent, tip vortex. While such vortex generation/deployable devicesare generally effective in reducing BVI noise, performance penaltiesand/or mechanical complexities are impediments to the widespreadacceptance of deployable passive systems.

Active systems effect noise control by continuously modifying the pitchor angle of attack of a rotor blade azimuthally about the rotationalaxis. This may be accomplished via selective control inputs by pitchcontrol actuators or through blade mounted control surfaces whichreceive control inputs from a closed- or open-loop feedback controlsystem. More specifically, the control system senses vibration/noise viaa plurality of accelerometers/microphones and provides higher ordercontrol inputs to the control actuators/control surfaces to pitch therotor blade at selected higher harmonic frequencies. The higher harmonicblade excursions effect vibration/noise reduction by influencing thetrajectory and/or strength of the interfering vortex, and/or producingpressure waves that directly cancel the BVI pressure impulse. Oneexample of an active system includes oscillating flaps disposed alongthe trailing edge of a rotor blade to provide a means of activelycontrolling the angle of attack of the rotor blade. Another exampleinvolves channeling air to the tip of a rotor blade and expelling suchair to disrupt the formation of the tip vortex. Active systems are,perhaps, the most effective in mitigating the BVI noise when compared topassive and deployable passive systems, however, active systems are themost disadvantageous in terms of incurred weight penalties, complexity,reliability and related fail-safe issues.

A need, therefore, exists to provide a means to significantly reduce theBVI noise radiated from rotor systems which does not significantlydegrade the operating efficiency thereof, e.g., lift capacity or powerrequirements, and does not significantly increase the weight, ormechanical complexity of the rotor system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a Variable DiameterRotor (VDR) system for reducing BVI noise which VDR system includestelescoping rotor blade assemblies and a positioning means forcontrolling the length of the rotor blade assemblies such that the rotorblade assemblies are fully-extended in a first operating mode foroptimal aerodynamic performance, and alternately vary in blade length ina second operating mode for optimal acoustic performance, i.e., BVInoise reduction.

It is yet another object of the present invention to provide a VDRsystem in combination with a tilt rotor aircraft for reducing BVI noisewhich VDR system includes telescoping rotor blade assemblies and apositioning means for controlling the length of the rotor bladeassemblies such that the rotor blade assemblies are extended in a firstoperating mode for optimal hover performance, alternately vary in bladelength in a second operating mode for optimal acoustic performance, andare retracted in a third operating mode for optimal forward flightperformance.

It is yet another object of the present invention to provide a VDRsystem for reducing BVI noise which VDR system includes telescopingrotor blade assemblies and a positioning means for controlling thelength of the rotor blade assemblies, which positioning means does notadversely impact the weight and/or mechanical complexity of the VDRsystem.

These and other objects are achieved by a Variable Diameter Rotor (VDR)system having telescoping odd and even blade assemblies wherein the oddblade assemblies define a radial length R_(O) and the even bladeassemblies define a radial length R_(E). Each blade assembly,furthermore, defines an internal chamber for accepting a positioningmeans operative to effect telescopic translation of the blade assembliessuch that the radial length R_(E) of the even blade assemblies is equalto the radial length R_(O) of the odd blade assemblies in a firstoperating mode, and such that the radial length R_(E) is between about70% to about 95% of the length R_(O) in a second operating mode.

The positioning means includes a centrifugal restraint assembly disposedwithin each internal chamber of the rotor blade assemblies, a stopsurface formed internally of each internal chamber, and an actuationmeans operative for transpositioning the centrifugal restraintassemblies within the internal chambers such that, in one operatingmode, the centrifugal restraint assemblies are disposed in abuttingengagement with the stop surfaces and, in another operating mode, theactuation means is disposed in abutting engagement with the centrifugalrestraint assemblies of at least one of the blade assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantfeatures and advantages thereof may be had by reference to the followingdetailed description of the invention when considered in conjunctionwith the following drawings wherein:

FIG. 1 is a top view of a VDR system in accordance with the presentinvention wherein odd and even rotor blade assemblies thereofalternately vary in length about the azimuth of the rotor system;

FIG. 2a depicts a two dimensional top view of the vortices shed by therotor blade assemblies of a conventional rotor system;

FIG. 2b is a section view along a vertical plane 2b of FIG. 2a,depicting the interaction of vortices of the conventional rotor system,and the spatial positioning of the vortices over time due to suchinteraction;

FIG. 2c depicts a vortex of the conventional rotor system interactingwith a blade element thereof;

FIG. 3a depicts a two dimensional top view of the vortices shed by therotor blade assemblies of the rotor system according to the presentinvention;

FIG. 3b is a section view along a vertical plane 3b of FIG. 3a,depicting the interaction of vortices of the conventional rotor system,and the spatial positioning of the vortices over time due to suchinteraction;

FIG. 3c depicts vortices of the rotor system according to the presentinvention and a blade element thereof passing above or below thevortices;

FIG. 4a is a graphical representation of an acoustic pressure profileillustrating the magnitude of BVI noise radiated from the conventionalrotor system;

FIG. 4b is a graphical representation of an acoustic pressure profileillustrating the magnitude of BVI noise radiated from the rotor systemof the present invention;

FIGS. 5a-5c depict a tilt rotor aircraft in combination with a pair ofVDR systems wherein the odd and even blade assemblies are extended in afirst operating mode (FIG. 5a), alternately vary in blade length in asecond operating mode (FIG. 5b) and are retracted in a third operatingmode (FIG. 5c);

FIG. 6a depicts a plan view of a VDR blade assembly having inboard andoutboard blade sections which are broken-away and sectioned to reveal aninternal positioning means for varying blade length;

FIG. 6b is a cross-sectional view taken substantially along line 6b--6bof FIG. 6a;

FIG. 6c is an enlarged view of the root end portion of the bladeassembly of FIG. 6a;

FIG. 6d is a cross-sectional view taken substantially along line 6d--6dof FIG. 6a;

FIG. 7 depicts an enlarged view of the relevant portions of thepositioning means including: a centrifugal restraint assembly disposedin combination with the outboard blade section and in combination withan internal stop surface, and an actuation means including a drive meansand a ballscrew assembly operative for engaging and disengaging thecentrifugal restraint assembly;

FIG. 8 is a partially broken-away side view of the drive means foreffecting rotation of the ballscrew assembly;

FIG. 9. depicts the positioning means in two operating positions whereina ball nut element of the ballscrew assembly is disengaged from thecentrifugal restraint assembly in one operating mode such that thecentrifugal restraint assembly engages the stop surface and wherein theball nut is engaged with the centrifugal restraint assembly in anotheroperating mode;

FIGS. 10a-10c schematically depict one embodiment of the positioningmeans in first, second and third operating modes wherein ballscrewassemblies are employed for effecting telescopic translation of the oddand even blade assemblies and wherein the relative position of the ballnuts effects the desired blade length variation;

FIGS. 11a-11c schematically depict another embodiment of the positioningmeans in the first, second and third operating mode wherein ballscrewassemblies are employed for effecting telescopic translation of the oddand even blade assemblies and wherein the relative position of the stopsurfaces effects the desired blade length variation;

FIG. 12a depicts a broken-away top view of a telescoping rotor bladewherein the positioning means employs a reeling assembly having endfixities for engaging or disengaging the centrifugal restraint assembly;

FIG. 12b is a cross-sectional view taken substantially along line12b--12b of FIG. 12a;

FIG. 12c is a cross-sectional view taken substantially along line12c--12c of FIG. 12a;

FIG. 12d depicts a broken-away and partially sectioned side view of thereeling assembly in combination with an overhead planetary gear systemfor driving the reeling assembly;

FIG. 12e is an enlarged view of the overhead planetary gear system whichis operative for driving a cylindrical drum of the reeling assembly;

FIG. 12f is a cross-sectional view taken substantially along lines12f--12f of FIG. 12d;

FIGS. 13a-13c schematically depict another embodiment of the positioningmeans in first, second and third operating modes wherein the reelingassembly is employed for effecting telescopic translation of the odd andeven blade assemblies and wherein the relative position of the endfixities effects the desired blade length variation;

FIGS. 14a-14c schematically depict yet another embodiment of thepositioning means in the first, second and third operating mode whereinthe reeling assembly is employed for effecting telescopic translation ofthe odd and even blade assemblies and wherein the relative position ofthe stop surfaces effects the desired blade length variation.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings wherein like reference characters identifycorresponding or similar elements throughout the several views, FIG. 1depicts a plan view of a four-bladed VDR system 4 having a rotor hubassembly 6 mounting to and driving odd and even rotor blade assembliesO_(b) and E_(b), respectively, about an axis of rotation 8. While thedescribed embodiment depicts a four-bladed VDR system 4, the inventionis also applicable to rotor systems having an even number of rotor bladeassemblies in excess of four, e.g., 6 or 8 rotor blade assemblies. Theodd and even blade assemblies O_(b), E_(b) of the VDR system 4 eachinclude inboard and outboard blade sections 10 and 12, respectively,wherein the outboard blade section 12 telescopically mounts to theinboard blade section 10. Furthermore, the telescoping rotor bladeassemblies O_(b), E_(b) alternate in radial length about the azimuth ofthe rotational axis 8 such that the tip path of the odd and even rotorblade assemblies O_(b) and E_(b) define two distinct rotor diametersD_(O), and D_(E), respectively. The positioning means for alternatingblade length are described in detail below, however, suffice it to saythat the telescopic translation of the odd and even blade assembliesO_(b), E_(b) may be controlled to vary the radial length of each.

Each odd blade assembly O_(b), defines a radial length R_(O) which maybe further defined as the fully-extended blade radius of the VDR system4, i.e., the maximum permissible length of the telescoping inboard andoutboard blade sections 10, 12. Each even blade assembly E_(b) defines aradial length R_(E) which may be further defined as a percentage of theradial length R_(O) of each odd blade assembly O_(b). Preferably, theradial length R_(E) is between a range of about 70% to about 95% ofradial length R_(O) and, more preferably, the radial length R_(E) isbetween about 82% to about 95% of radial length R_(O). The functionalsignificance of the selected ranges will become apparent in view of thefollowing discussion.

In FIGS. 2a-2c and FIGS. 3a-3c, the BV Interactions of a conventional,uniform blade length rotor system 14 are compared and contrasted withthose of the alternating blade length VDR system 4 of the presentinvention. In FIG. 2a, the rotor blades B₁ -B₄ of the conventional rotorsystem 14 produce vortices V₁ -V₄ which are plotted over a period of onerevolution. As shown, the rotor system 14 is moving forward at avelocity V_(F) and in a descending flight profile. The two dimensionalvortex plot shows a high concentration of vortices V₁ -V₄ in the firstquadrant of the rotor system 14 i.e., between 0 and 90 degrees. In FIG.2b, the vortices V₁ -V₄, are shown in section through a vertical plane2b located in the second quadrant. Initially, the vortices V₁ -V₄ areabove the plane XY of the rotor, and are moving downward at velocityV_(Z). Furthermore, the vortices V₁ -V₄, are rotating in a clockwisedirection and are substantially equally-spaced with respect to oneanother i.e., in both horizontal and vertical directions. As timeadvances, the vortices V₁ -V₄ interact such that they remain insubstantially collinear alignment, i.e., along line L_(V), as thevortices V₁ -V₄ progress in a downward direction. That is, thealternating upward and downward circulation of the vortices V₁ -V₄produce induced velocity vectors I_(v) which are substantially equal andopposite, thereby maintaining the vertical spacing between the vorticesV₁ -V₄. Such uniform vertical spacing creates a high probability that asubsequent rotor blade will interact with one of the Vortices V₁ -V₄.FIG. 2c depicts a typical BV interaction in the first quadrant whereinthe Vortex V₂ is in close proximity to a blade element B_(e) of rotorblade B₁ and is substantially parallel thereto. As discussed in the"Background of the Invention" such close proximity, parallelinteractions result in high BVI noise.

In FIGS. 3a-3c, the odd and even rotor blade assemblies O_(b) and E_(b)of the VDR system 4 according to the present invention produce vorticesV_(O) and V_(E), respectively. The same flight conditions and views areshown as those depicted in FIGS. 2a-2c, however, the trajectory andspatial separation of the vortices V_(O) and V_(E) are altered due tothe variation of blade length. More specifically, and referring to FIG.3b, the vortices V_(O), V_(E) are initially clustered in closely spacedpairs P₁ and P₂, wherein the pairs P₁, P₂ are distally spaced in ahorizontal direction. As time advances, the vortices V_(O), V_(E) ofeach pair P₁, P₂ interact to effect an increased spatial separation in avertical direction. That is, the horizontal separation of the vortexpairs P₁, P₂ causes the induced velocity vectors I_(v) to influence eachpair P₁, P₂ independently, i.e., without influencing each other. Theinduced velocity I_(v) causes the vortices V_(O), V_(E) of each pair toseparate vertically, i.e., by impeding the downward progression ofvortices V_(E) while accelerating the downward motion of vortices V_(O).While it may have been expected that the vortices V_(O), V_(E) of eachpair P₁, P₂ should rotate about one another without effecting a verticalseparation, additional factors such as the proximity of each vortex pairP₁, P₂ to prior generated vortices and the curvature/orientation of thevortices V_(O), V_(E) combine to produce the phenomena described above.As shown in FIG. 3c, the spatial separation causes the blade elementB_(e) to pass above or below the vortices V_(O), V_(E) by a distanceR_(s), wherein R_(s) is at least 5X the thickness of the blade element.As discussed earlier, such spatial separation results in substantiallybenign BV Interactions.

FIGS. 4a and 4b graphically depict the acoustic pressure profiles of theconventional rotor system 14 and the VDR system 4 of the presentinvention. In FIG. 4a, the conventional rotor system 14 produces rapidpressure fluctuations 18 having min and max values from about -40 PA(-8.4 lb/ft²) to about +40 PA (+8.4 lb/ft²), respectively, over a 1-3msec time interval. The pressure fluctuations 18 are indicative of highimpulsive pressure waves which are the source of BVI noise. Secondaryspikes 20 are indicative of pressure waves generated by blade loading,e.g., lift, and are less offensive to the human ear due to the longertime interval, i.e., 20-25 msec, over which such fluctuations occur. InFIG. 4b, the VDR system of the present invention produces substantiallyreduced BVI impulses 24 which fluctuate from about -10 PA (-2.2 lb/ft²)to +10 PA (+2.2 lb/ft²). Secondary spikes 26 and 28 are indicative ofthe blade loading on the even and odd blade assemblies O_(b) and E_(b),respectively, and fluctuate in magnitude due to the alternating bladelength. When comparing the BVI components of pressure in FIGS. 4a and4b, and converting the same to the frequency domain, a 10-16 dBareduction in BVI noise is achieved when employing the teachings of thepresent invention.

The VDR system of the present invention may be reconfigured in variousmodes of flight operation for maximum aerodynamic and acousticperformance. For example, all blade assemblies O_(b), E_(b) thereof maybe fully-extended and uniform in length, in a first operating modecorresponding to hover or forward flight operations. In thisconfiguration, the rotor disk area is maximized for optimum aerodynamicperformance. Furthermore, the blade assemblies O_(b), E_(b) may beselectively positioned for alternating blade length in a secondoperating mode corresponding to descending flight operations. In thisconfiguration, BVI noise is mitigated for maximum acoustic performance.

APPLICATION TO TILT ROTOR AIRCRAFT

While the present invention is useful in combination with VDR systems ingeneral, its application to tilt rotor aircraft provides a considerableopportunity for BVI noise reduction. A tilt rotor aircraft is a type ofrotorcraft which employs a pair of rotor systems which are supported atthe outermost end of a wing structure and are pivotable such that therotors thereof may assume a vertical or horizontal orientation. In ahorizontal orientation, the aircraft is capable of hovering flight,while in a vertical orientation, the aircraft is propelled in the samemanner as conventional propeller-driven fixed-wing aircraft.

Currently, tilt rotor aircraft employ conventional fixed-diameter rotorsystems which, in the aerodynamic, acoustic and aeroelastic designthereof, attempt to blend the competing requirements of hovering andforward flight modes of operation. For example, with regard to hoveringflight, it is generally advantageous to employ a large diameter rotor toimprove hovering performance by lowering disk loading, reducing BVInoise levels, and reducing downwash velocities. Conversely, a relativelysmall diameter rotor is desirable in forward flight to improvepropulsive efficiency by minimizing blade aeroelastic properties,minimizing blade area, and reducing tip speed (Mach number). Within theconstraints of a fixed-diameter rotor system, these requirementsnecessitate that design compromises be made which result in non-optimumaerodynamic and acoustic performance. With regard to acousticperformance, high blade loading in descending flight modes produceshighly vexatious BVI noise.

Variable Diameter Rotor (VDR) systems are known to provide distinctadvantages over conventional fixed-diameter rotors insofar as suchsystems are capable of adaptation to both modes of operation. That is,when the plane of the rotor is oriented horizontally, the rotor diameteris enlarged for improved hovering efficiency and, when orientedvertically, the rotor diameter is reduced for improved propulsiveefficiency. Tilt rotor aircraft which employ VDR systems are describedand depicted in U.S. Pat. Nos. 3,768,923, 4,142,697, and 5,253,979. VDRsystems for use in combination with tilt rotor aircraft provide areduction in blade loading and, consequently, a degree of BVI noisereduction. Further improvements, however, are achievable when employingthe teachings of the present invention.

The subsequent discussion will address various positioning means forvarying the length of VDR blade assemblies. The mechanisms are usefulfor any variety of VDR system, but are described in the context of atilt rotor aircraft wherein all the blade assemblies may be extended,alternately varied in length, or retracted. More specifically, andreferring to FIGS. 5a-5c, the blade assemblies O_(b), E_(b) of the VDRsystems 4 are extended in a first operating mode corresponding to hoveror low speed flight operations (FIG. 5a), are alternately varied inblade length in a second operating mode corresponding to descendingflight operations (FIG. 5b), and are retracted in a third operating modecorresponding to high speed cruise flight operations (FIG. 5c).

BALLSCREW ARRANGEMENT FOR VARYING BLADE LENGTH

Inasmuch as the telescoping features and blade positioning means aresimilar for all blade assemblies O_(b), E_(b), it will facilitate thediscussion to describe one of the blade assemblies, e.g., the odd bladeassembly O_(b), with the objective of describing those features whichare common to both odd and even blade assembles O_(b), E_(b).Subsequently, the discussion will distinguish between the odd and evenrotor blade assemblies O_(b) and E_(b), to identify structural andfunctional differences therebetween which facilitate control of rotorblade length.

In FIG. 6a, the odd blade assembly O_(b) is broken away to reveal theinternal arrangement for varying blade length. The outboard bladesection 12 telescopically mounts to the inboard blade section 10(hereinafter referred to as a torque tube member). More specifically,the outboard blade section 12 includes a spar member 34 which isenveloped by a leading edge sheath 36 (FIG. 6b) and a foam-filledtrailing edge pocket assembly 38 to define the requisite aerodynamiccontour of the outboard blade section 12. The spar member 34 iscoaxially aligned with and accepts the torque tube member 10 so as topermit telescopic translation thereof relative to the torque tube member10.

Coaxial alignment of the torque tube and spar members 10, 34 may beeffected by any conventional bearing assembly such as a rolling elementbearing assembly, however, in the described embodiment, the bearingassembly includes a first bearing block 40a mounted to the outboard endof the torque tube member 10 and a second bearing block 40b mounted tothe inboard end of the spar member 34, and preferably internallythereto. The first bearing block 40a slideably engages an internal pilotsurface 42 formed within the spar member 34 and the second bearing block40b slideably engages an external pilot surface 44 formed about theexterior of the torque tube member 10.

In FIG. 6c, the root end of the torque tube member 10 is adapted formounting to a cuff assembly 50 which is journally mounted to a radialarm 52 of the rotor hub assembly 6. The cuff assembly 50 engages aflared root end portion 54 of the torque tube member 10 by means of acorrespondingly shaped internal restraint member 56 and an externalrestraint sleeve 58. The cuff assembly 50, furthermore, includes a pitchcontrol arm 60 through which pitch control inputs are made about thefeathering axis 62 of the blade assembly O_(b).

In FIG. 7, the torque tube and spar members 10, 34 define an internalchamber 64 for accepting a blade positioning means 70. The positioningmeans 70 is operative to position the outboard blade section 12 relativeto the torque tube member 10 and, consequently, vary the radial lengthof the blade assembly O_(b). The positioning means 70 includes: acentrifugal restraint assembly 80, a stop surface 64s formed internallyof the chamber 64, and an actuation means 90 operative for engaging anddisengaging the centrifugal restraint assembly 80.

The centrifugal restraint assembly 80 includes a retention block 82, acradle member 84 and a connecting means 86 disposed between andmechanically coupling the retention block 82 to the cradle member 84.The retention block 82 is disposed in combination with the outboardblade section 12 and is operative, in combination with the connectingmeans 86, for transferring centrifugal loads Cf acting on the outboardblade section 12 to the cradle member 84. A retention block of the typedescribed is fully disclosed in co-pending, commonly owned U.S. patentapplication Ser. No. 08/412,035 (therein referred to as a "restraintassembly"). Preferably, the connecting means 86 for coupling theretention block 82 to the cradle member 84 includes fore and aft cablemembers 86a and 86b, respectively, which are compliant for accommodatingvarious motions of the rotor blade assembly O_(b).

The cradle member 84 is a generally U-Shaped member having radiallyextending arms 84a and a base portion 84b for structurallyinterconnecting the inboard ends of the radial arms 84a. The radial arms84a are mechanically coupled to the connecting means 86 and are disposedin sliding combination with an internal wall 64w of the chamber 64 forpermitting radial translation of the cradle member 84 therein.

The actuation means 90 includes a ballscrew assembly 92 disposed withinthe internal chamber 64, and a drive means 100 disposed internally ofthe rotor hub assembly 6. More specifically, the ballscrew assembly 92includes a threaded ballscrew 94 which extends through an aperture 84h(FIG. 6b) formed in the base portion 84b of the cradle member 84 and isoperative for rotation in a clockwise or counterclockwise directionabout its longitudinal axis 94L. The ballscrew 94 is supported at aninboard end by a first journal bearing 96a (FIG. 6c) and, at an outboardend, to a second journal bearing 96b (FIG. 6a) disposed in combinationwith the outboard end of the torque tube member 10. The actuation means90 further includes a ball nut 98 which is disposed in combination withthe threads of the ballscrew 94, and positioned radially outboard of thecradle member 84. Furthermore, the ball nut 98 is operative, in responseto rotation of the ballscrew 94, to translate axially along thelongitudinal axis 94L thereof. Rotational restraint of the ball nut 98is provided by the internal geometry of the torque tube member 10 (FIG.6d). Ballscrew assemblies of the type described are available fromThomson Saginaw, located in Saginaw, Mich.

The drive means 100 effects rotation of the ballscrew 94 and,consequently, axial displacement of the ball nut 98. In FIG. 8, thedrive means 100 includes an input shaft 102 disposed internally of andcoaxial with the main rotor shaft 104 which supports and drives therotor assembly 6 about its rotational axis 8. The input shaft 102 drivesan input bevel gear 106i through a first universal joint 108-1 whichaccommodates gimbal tilt motion of the rotor hub assembly 6. The inputbevel gear 106i drives an output bevel gear 106o, one per bladeassembly, which is coupled, via a second universal joint 108-2 to theballscrew 94. The ballscrew 94 may be driven in either direction or,alternatively, rotationally fixed by controlling the rotational speed ofthe input shaft 102 relative to the main rotor shaft 104. For example,by driving the input shaft 102 and, consequently, the input bevel gear106i, at a higher rotation speed than the main rotor shaft, the outputbevel gear 106o, which is rotating with the main rotor shaft 104, willbe driven in the direction of the speed differential. If the input driveshaft 102 rotates at the same operational speed as the main rotor shaft104, no differential rotation is effected between the input and outputbevel gears 106i, 106o, hence the ballscrew 94 will remain rotationallyfixed. Drive means which are structurally and functionally similar tothat described above are discussed in U.S. Pat. Nos. 4,142,697,4,009,997, 3,884,594 and 3,713,751.

In FIG. 9, the cradle member 84 is outwardly biased by centrifugalforces Cf acting on the outboard blade section 12 such that the cradlemember 84 is disposed in abutting engagement with the ball nut 98 or thestop surface 64s to determine the radial length of the rotor bladeassembly O_(b). In one of the operating modes, the cradle member 84engages the stop surface 64s vis-a-vis a first bearing surface 84s-1thereof such that the rotor blade assembly O_(b) is fully-extended,i.e., at its maximum permissible length. Centrifugal loads Cf acting onthe outboard blade section are transferred to the cradle member 84 andreacted by the stop surface 64s. In another operating mode, the ball nut98 is transpositioned, in response to rotation of the ballscrew 94, soas to engage the cradle member 84 vis-a-vis a second bearing surface84s-2 thereof. In this operating mode, the cradle member 84 follows theball nut 98 provided, however, that the first bearing surface 84s-1 isdisengaged from the stop surface 64s. Furthermore, centrifugal loadsacting on the outboard blade section 12 are transferred to the actuationmeans 90 via the cradle member 84. The outboard blade section 12 isthereby caused to telescope inwardly or outwardly depending upon thedirectional displacement of the ball nut 98.

The foregoing discussion has described the structural elements which arecommon to all blade assemblies O_(b), E_(b) of the VDR system 4. Thefollowing discussion addresses the structural or functional differencesbetween odd and even blade assemblies O_(b), E_(b) which produce thedesired blade length variations. A subscript "O" or "E" will be used todistinguish between odd to even blade assemblies, O_(b) to E_(b).

In FIGS. 10a-10c, one embodiment of the present invention is shownwherein the positioning means 70 for controlling blade length isschematically depicted. Each schematic depicts the first, second, andthird operating modes corresponding to FIGS. 5a, 5b and 5c,respectively. Representative odd and even blade assemblies O_(b), E_(b)are arranged in side-by-side relation for ease of comparison. Moreover,the external structures which envelop the positioning means 70, e.g.,the spar member 34, torque tube member 10, rotor hub assembly 6 etc.,have been removed to facilitate the discussion.

While many elements of the positioning means 70 have been previouslydescribed in the context of the odd blade assembly O_(b), it should beunderstood that the positioning means 70 controls the blade length ofall blade assemblies, i.e., odd and even blade assemblies O_(b), E_(b).In FIG. 10a, the ballscrews 94_(O), 94_(E) are supported between thefirst and second bearing assemblies and extend through the respectivecradle members 84_(O), 84_(E) such that the ball nuts 98_(O), 98_(E) aredisposed radially outboard thereof. Furthermore, the ball nuts 98_(O)and 98_(E) are positioned along the respective ballscrews 94_(O) and94_(E) so as to define radial distances Rn_(O) and Rn_(E), respectively,measured from the rotational axis 8 of the rotor hub assembly.Initially, the relative position of the ball nuts 98_(O), 98_(E) isprescribed such that the ball nut 98_(E) of the even blade assemblyE_(b) is radially inboard of the ball nut 98_(O) of the odd bladeassembly O_(b). The functional significance of such relative positionwill become apparent in the subsequent views.

In the first operating mode, the ballnuts 98_(O), 98_(E) are disengagedfrom the cradle members 84_(O), 84_(E) such that the cradle members84_(O), 84_(E), in response to centrifugal loads C_(f), are disposed inabutting engagement with the stop surfaces 64s_(O), 64s_(E). Insofar asthe length of the centrifugal restraint assemblies 80_(O), 80_(E) ispreferably constant i.e., from odd to even blade assemblies O_(b),E_(b), the radial distances Rs_(O), Rs_(E) defined by the stop surfaces64s_(O), 64s_(E) determine the maximum permissible length R_(O), R_(E)of the odd and even rotor blade assemblies O_(b), E_(b). In thedescribed embodiment, the radial distance Rs_(O) of the odd bladeassembly O_(b) is equal to the radial distance Rs_(E) of the even bladeassembly E_(b), such that blade assemblies O_(b), E_(b) arefully-extended and equal in radial length. Such blade assemblyconfiguration provides maximum rotor disk area for optimum aerodynamicperformance.

In the second operating mode (FIG. 10b), the ball nuts 98_(O) and 98_(E)are transpositioned inwardly to an intermediate position in response torotation of the respective ballscrews 94_(O), 94_(E) by the drive means100. Inasmuch as the ball nut 98_(E) is disposed radially inboard of theball nut 98_(O), the ball nut 98_(E) engages the respective cradlemember 84_(E) thereby effecting inward telescopic translation of theoutboard blade section 12_(E). The ball nut 98_(O) of the odd bladeassembly O_(b) also translates inwardly, but does not engage therespective cradle member 84_(O). When disposed in their respectiveintermediate positions, the ball nut 98_(E) is disposed in abuttingengagement with the respective cradle member 84_(E) and the ball nut98_(O) is disengaged from the respective cradle member 84_(O).Consequently, the cradle member 84_(O) remains in abutting engagementwith the stop surface 64s_(O).

In view of the foregoing, it will be appreciated that the radialdistances Rn_(O), Rn_(E) defined by the ball nuts 98_(O), 98_(E)determines the maximum permissible variation in blade assembly lengthR_(O), R_(E). That is, by suitably positioning the ball nuts 98_(O),98_(E) along the respective ballscrews 94_(O), 94_(E), the length R_(E)of the even blade assembly E_(b) may be altered without influencing theradial length R_(O) of the odd blade assembly O_(b). More specifically,the ball nuts 98_(O), 98_(E) are positioned such that radial distanceRn_(E) of ball nut 98_(E) is between about 0.7Rn_(O) -0.3L to about0.95Rn_(O) -0.05L and, preferably, between about 0.83Rn_(O) -0.17L toabout 0.92Rn_(O) -0.O8L, wherein L is the length from one of the cradlemembers 84_(O), 84_(E) to the tip end of the one of the outboard bladesection 12_(O), 12_(E). This is also based on the assumption that thelengths L from odd to even blade assemblies O_(b), E_(b) are equal.Consequently, the odd and even blade assemblies O_(b), E_(b), may bevaried in length by an amount proportional to the radial spacing of theball nuts 98_(O), 98_(E). This blade assembly configuration is effectedin a descending flight mode for optimum acoustic performance, i.e., BVInoise reduction.

In the third operating mode (FIG. 10c), the ball nuts 98_(O) and 98_(E),and consequently, the cradle members 84_(O), 84_(E), are transpositionedinwardly to a fully-inboard position. During the transition, the ballnut 98_(O) of the odd blade assembly O_(b) engages the respective cradlemember 84_(O) and translates inwardly at an increased linear raterelative to the ball nut 98_(E). The linear rate differential causes theball nuts 98_(O) and 98_(E) to reach their respective inboard positionsat the same time, i.e., such that the radial distance Rn_(E) equals theradial distance Rn_(O). Such rate differential may be effected byvarying the thread pitch of the ballscrews 94_(O), 94_(E), varying therotational speed of the ballscrews 94_(O), 94_(E), or a combinationthereof. With regard to the former, a thread pitch differential may beeffected by increasing the thread pitch of the ballscrews 94_(O) withrespect to the thread pitch of the ballscrews 94_(E). The pitchdifferential causes the ball nuts 98_(O), 98_(E) to traverse atdifferent linear rates such that at an inboard radial position, theradial distances Rn_(O), Rn_(E) of the ball nuts 98_(O), 98_(E) areequal. With regard to the latter, a speed differential may be effectedby altering the gear ratios of the input and output bevel gears 106i,106o_(O), 106o_(E) such that the rotational speed of ballscrew 94_(O) isgreater than the rotational speed of ballscrew 94_(E).

When in their respective inboard positions, the ball nuts 98_(O), 98_(E)are disposed in abutting engagement with the cradle members 84_(O),84_(E) and the blade assemblies O_(b), E_(b) are fully-retracted. Suchblade assembly configuration corresponds to high speed cruise flightoperations wherein the rotor disk is vertically oriented for maximumpropulsive efficiency.

Another embodiment of the positioning means 70 is shown in FIGS. 11a-11cwherein the same operating modes are depicted as those illustrated anddescribed in FIGS. 10a-10c. In this embodiment of the invention, theradial distances Rn_(O), Rn_(E) defined by the ball nuts 98_(O), 98_(E)are equal, and the radial distances Rs_(O), Rs_(E) defined by the stopsurfaces 64s_(O), 64s_(E) determine the percentage variation in bladelength R_(O), R_(E). In the first operating mode, (FIG. 11a) ballnut98_(O) of the odd blade assembly 0_(b) is disposed in abuttingengagement with the cradle member 84_(O) and the ball nut 98_(E) of theeven blade assembly E_(b) is disengaged from the cradle member 84_(E)thus causing the cradle member 84_(E) to be disposed in abuttingengagement with the stop surface 64s_(E). The radial distances Rn_(O)and Rs_(E) defined by the ball nut 84_(O) and the stop surface 64s_(E),respectively, are substantially equal such that the odd and even bladeassemblies O_(b), E_(b) are extended, and the radial length R_(O) of theeven blade assembly O_(b) is equal to the radial length R_(E) of theeven blade assemblies E_(b). These radial distances Rn_(O), Rs_(E) mayvary depending upon the configuration of the cradle member 84_(O),84_(E).

In the second operating mode, (FIG. 11b) the ball nuts 98_(O), 98_(E)are transpositioned to an outboard position such that the cradle member84_(O) of the odd blade assembly O_(b) also engages the respective stopsurface 64s_(O). Consequently, the odd blade assembly O_(b) willtranslate radially outboard to its fully-extended position while theeven blade assembly E_(b) remains fixed due to the axial restraintprovided by the stop surface 64s_(E). In this embodiment of theinvention, the maximum permissible variation in blade assembly lengthR_(O) and R_(E) is effected by varying the radial distances Rs_(O),Rs_(E) of the stop surfaces 64s_(O), 64s_(E). More specifically, thestop surfaces 64s_(O), 64s_(E) are positioned such that the radialdistance Rs_(E) is between about 0.7Rs_(O) -0.3L to about 0.95Rs_(O)-0.05L and, preferably, between about 0.83Rs _(O) -0.17L to about0.92Rs_(O) -0.08L, wherein L is the length from one of the cradlemembers 84_(O), 84_(E) to the tip end of the one of the outboard bladesection 12_(O), 12_(E). Again, it is assumed that the length L is equalfrom odd to even blade assemblies O_(b), E_(b). Consequently, the oddand even blade assemblies O_(b), E_(b) may be varied in length by anamount proportional to the radial spacing of the stop surfaces 64s_(O),64s_(E).

In the third operating mode (FIG. 11c), the ball nuts 98_(O), 98E engagethe cradle members 84_(O), 84_(E) and translate to a fully inboardposition. During the transition from the second to third operatingmodes, the ball nut 98_(O) of the odd blade assembly O_(b) initiallyengages the respective cradle member 84_(O) and, subsequently, the ballnut 98_(E) of the even blade assembly engages its respective cradlemember 84_(E). With both cradle members 84_(O), 84_(E) engaged, theoutboard blade sections 12_(O), 12_(E) telescope inwardly, in unison, tothe desired fully-inboard position. When in their respective inboardpositions, the ball nuts 98_(O), 98_(E) are disposed in abuttingengagement with the cradle members 84_(O), 84_(E) and the bladeassemblies 0_(b), E_(b) are fully-retracted.

While the described embodiment employs an actuation means 70 having aballscrew assembly 92 for effecting telescopic translation of the bladeassemblies O_(b), E_(b), it will be appreciated that other devices suchas a threaded jackscrew/nut or a threaded rollerscrew/roller nut may besubstituted therefor. Furthermore, while the described embodimentemploys a cradle member 84 having a generally U-shape, it will beappreciated that the configuration of the cradle member may take anyform. For example, a simple cross member which conforms to the shape ofthe internal chamber 64 may functionally replace the U-shaped cradlemember 84. While the described embodiment includes cable members 86a,86b for connecting the retention block 82 to the cradle members 84, itwill be appreciated that other means are contemplated. For example,strap or compliant tubular members which are capable of withstandinghigh tensile loads may be employed in lieu of the cable members 86a,86b.

REELING ASSEMBLY FOR VARYING BLADE LENGTH

Other embodiments of the positioning means 70 are shown in FIGS.12a-12f, 13a-13c, and 14a-14c wherein the actuation means 90 includes areeling assembly 110 and means 120 for driving the reeling assembly 110.Again, it will facilitate the discussion to describe the positioningmeans 70 with respect to one blade assembly and subsequently discuss thefeatures thereof which permit independent control of odd and even bladeassemblies for varying blade length.

In FIGS. 12a-12c, the reeling assembly 110 includes a cylindrical drum112 operative for rotation about the rotor hub assembly axis 8, an endfixity 114 disposed internally of the chamber 64 and operative forengaging and disengaging the cradle member 84, and strap means 116 forconnecting the end fixity 114 to the cylindrical drum 112. Morespecifically, the end fixity 114 is disposed radially outboard of thecradle member 84 and disposed in sliding combination with the connectingmeans 86 (FIG. 12b) and/or an internal wall 64ws of the chamber. Thestrap means 116 is mechanically coupled to the end fixity 114 andextends through the aperture 84h of the cradle member 84 (FIG. 12c) forconnecting to the cylindrical drum 112. Depending upon the rotationalsense of the cylindrical drum 112, the strap means 116 is wound-up orlet-out such that the end fixity 114 is operative to engage the cradlemember 84 or, alternatively, disengage from the cradle member 84 suchthat the cradle member engages the stop surface 64s. When the end fixity114 is engaged with the cradle member 84, centrifugal loads C_(f) actingon the outboard blade section 12 are transferred to the cylindrical drum112.

In FIG. 12d-12f, the cylindrical drum 112 is disposed internally of therotor hub assembly 6 and may be driven in either direction by the drivemeans 120. The drive means 120 includes an overhead planetary gearsystem 140 which is driven by an input drive shaft 122. Morespecifically, the input drive shaft 122 includes: an upper stub shaftportion 122u which is supported by an upper mounting fixture 126 of therotor hub assembly 6 via a bearing assembly 128, a lower drive shaftportion 122L coaxially aligned with the main rotor shaft 104, and auniversal joint 130 disposed therebetween for accommodating gimbal tiltmotion of the rotor hub assembly 6. The cylindrical drum 112 issupported for rotation about the stub shaft portion 122u via a bearingassembly 132, thereby permitting the drum 112 to synchronously tilt withthe rotor hub assembly 6.

The planetary gear system 140 includes: a driving sun gear 142 coaxiallyaligned with the rotor hub assembly axis 8, a ring gear 144 rigidlyaffixed to the mounting fixture 126 of the rotor hub assembly 6, and aplurality of planetary pinions 146 disposed between and interacting withthe sun and ring gears 142, 144. The sun gear 142 is spline connected toand driven by the stub shaft portion 122u of the input drive shaft 122and drives the planetary pinions 146 about the rotor hub assembly axis8. More specifically, the planetary pinions 146 traverse in an epicycleabout the sun gear 142 at reduced rotational speed relative thereto dueto interaction of the planetary pinion 146 with the ring gear 144. Theoutput of the planetary pinions 146 is transferred to the cylindricaldrum 112 by means of carrier posts 148 which are disposed in combinationwith each planetary pinion 146 and the upper surface 150 of thecylindrical drum 112.

It will be appreciated that the sum of the centrifugal loads C_(f)acting on the outboard blade sections 12 is reacted in torsion by theinput drive shaft 122. By interposing the planetary gear system 140 inthe load path, i.e., from the outboard blade section 12 to the inputdrive shaft 122, the torsional load imposed on the input drive shaft 122is substantially reduced. That is, the planetary gear system 140,provides a mechanical advantage which substantially reduces thetangential loading on the sun gear 142, and, accordingly, the torsionalloads on the input drive shaft 122. Consequently, the requisite sizeand/or thickness of the input drive shaft 122 may be reduced. In thedescribed embodiment, the pitch diameter ratio Rs/Rp between the sun andplanetary gears 142, 146 is about 0.75 and the face width, i.e., theheight dimension, of the meshing teeth is about 7.5 in (19 cm). Thiscombination of parameters reduces the torsional loading in the inputdrive shaft 122 by a factor of 4.

In FIGS. 13a-13c, an embodiment of the positioning means 70/reelingassembly 110 is schematically depicted, which embodiment isoperationally similar to FIGS. 10a-10c. Similar to the earlier describedembodiments, a subscript "O" and "E" will be used to differentiatebetween odd and even blade assemblies O_(b), E_(b).

In FIG. 13a, the cylindrical drum 112 of the reeling assembly 110includes first and second cylindrical surfaces 112-1 and 112-2,respectively, which are disposed in biplanar relation. The first andsecond cylindrical surfaces 112-1 and 112-2 define diameter dimensionsDc1 and Dc2, respectively, wherein the diameter Dc1 of the firstcylinder 112-1 is greater than the diameter Dc2 of the second cylinder112-2. Furthermore, the strap means 116_(O) and 116_(E) of the odd andeven blade assemblies O_(b) and Eb, respectively, are disposed inwinding combination with the first and second cylinders 112-1 and 1122,respectively. The length of the strap means 116_(O), 116_(E), isprescribed such that the end fixities 114_(O), and 114_(E) define radialdistances Rf_(O), and Rf_(E), respectively, from the rotational axis 8of the rotor hub assembly. Initially, the radial distance Rf_(O) isgreater than radial distance Rf_(E).

In the first operating mode, the end fixities 114_(O), 114_(E) aredisengaged from the cradle members 84_(O), 84_(E), such that the cradlemembers 84_(O), 84_(E) are disposed in abutting engagement with the stopsurfaces 64s_(O), 64s_(E). In the described embodiment, the radialdistances Rs_(O), Rs_(E) defined by the stop surfaces 64s_(O), 64s_(E)are equal, hence the blade assemblies O_(b), E_(b) are equal in radiallength and are fully extended.

In the second operating mode (FIG. 13b), the end fixities 114_(O),114_(E) are transpositioned inwardly to an intermediate position byrotation of the cylindrical drum 112 and wind-up of the respectivelystrap means 116_(O), 116_(E). During the transition, the end fixity114_(E) of the even blade assembly E_(b) engages the respective cradlemember 84_(E) thereby effecting inward telescopic translation of theoutboard blade section 12_(E). The end fixity 114_(O) of the outboardblade section O_(b) also translates inwardly, but does not engage therespective cradle member 84_(O). When disposed in their respectiveintermediate positions, the end fixity 114_(E) is disposed in abuttingengagement with the respective cradle member 84_(E) and the end fixity114_(O) is disengaged from the respective the cradle member 84_(O) suchthat the cradle member 84_(O) remains in abutting engagement with thestop surface 64s_(O). Furthermore, the end fixities 114_(O), 114_(E) arepositioned such that the radial distance Rf_(E) of the end fixity114_(E) is between about 7Rf_(O) -0.3L to about 0.95Rf₀ -0.05L and,preferably, between about 0.83Rf_(O) -0.17L to about 0.92Rf_(O) -0.08L,wherein L is the length from one of the cradle members 84_(O), 84_(E) tothe tip end of the one of the outboard blade section 12_(O), 12_(E).Such relative position of the end fixities 114_(O), 114_(E) effects themaximum permissible variation in blade length R_(O), R_(E).

In the third operating mode, (FIG. 13c), the end fixities 114_(O),114_(E) engage the cradle members 84_(O), 84_(E) and are transpositionedinwardly to a fully inboard position. Insofar as the diameter Dc1 of thefirst cylindrical surface 112-1 is greater than the diameter Dc2 of thesecond cylindrical surface, the end fixity 114_(O) of the odd bladeassembly O_(b) will translate inwardly at an increased linear raterelative to the end fixity 114_(E). The linear rate differential causesthe end fixities 114_(O), 114_(E) to reach their respective inboardpositions at the same instant in time, i.e., such that the radialdistance Rf_(O) is equal to the radial distance Rf_(E).

In FIGS. 14a-14c, another embodiment of the positioning means 70/reelingassembly 110 is schematically depicted, which embodiment isoperationally similar to FIGS. 11a-11c. In FIG. 14a, the cylindricaldrum 112 defines a cylindrical surface 112s upon which the strap means116_(O), 116_(E) may be wound on or off. The strap means 116_(O),116_(E) are of equal length, and, consequently, the end fixities114_(O), 114_(E) are equidistant from the rotational axis 8.Furthermore, the stop surfaces 64s_(O) and 64s_(E) define radialdistances Rs_(O) and Rs_(E), respectively, wherein the radial distanceRs_(E) is between about 0.7Rs_(O) -0.3L to about 0.95Rs_(O) -0.05L, and,preferably, between about 0.83Rs_(O) -0.17L to about 0.95Rs_(O) -0.O8Lwherein L is the length from one of the cradle members 84_(O), 84_(E) tothe tip end of the one of the outboard blade section 12_(O), 12_(E).

In the first operating mode, the end fixity 114_(O) of the odd bladeassembly O_(b) is disposed in abutting engagement with the cradle member84_(O) and the end fixity 114_(E) of the even blade assembly E_(b) isdisengaged from the cradle member 84_(E). Consequently, the cradlemember 84_(E) is disposed in abutting engagement with the stop surfaces64s. The radial distances Rf_(O) and Rs_(E) defined by the end fixity114_(O) and the stop surface 64s_(E), respectively, are substantiallyequal such that the odd and even blade assemblies O_(b), E_(b) areextended, and the radial length R_(O) of the even blade assembly O_(b)is equal to the radial length R_(E) of the even blade assemblies E_(b).

In the second mode, (FIG. 14b), the end fixities 114_(O), 114_(E) aretranspositioned to an outboard position, in response to rotation of thecylindrical drum 112, such that the cradle member 84_(O) of the oddblade assembly O_(b) also engages the respective stop surface 64s_(O).Consequently, the odd blade assembly O_(b) will translate to its fullyextended position while the even blade assembly E_(b) remains fixed dueto the axial restraint provided by the stop surface 64s_(E). In thisembodiment of the invention, maximum permissible variations in bladeassembly length R_(O) and R_(E) may be effected by varying the radialdistances Rs_(O), Rs_(E) of the stop surfaces 64s_(O) and 64s_(E),respectively.

In FIG. 14c, the end fixities 114_(O), 114_(E) engage the cradle members84_(O), 84_(E) and translate to a fully inboard position. Insofar as theend fixities 114_(O), 114_(E) are equidistant from the rotational axis8, the end fixity 114_(O) of the odd blade assembly O_(b) initiallyengages the respective cradle member 84_(O) and, subsequently, the endfixity 114_(E) of the even blade assembly engages its respective cradlemember 84_(E). With both cradle members 84_(O), 84_(E) engaged, theoutboard blade sections 12_(O), 12_(E) telescope inwardly,synchronously, to the desired inboard position. When in their respectiveinboard positions, the end fixities 114_(O), 144_(E) are disposed inabutting engagement with the cradle members 84_(O), 84_(E) and the bladeassemblies O_(b), E_(b) are fully retracted.

The various embodiments of the positioning means 70 have minimal impacton the weight and/or mechanical complexity of the telescoping bladeassemblies O_(b), E_(b). That is, the introduction of stop surfaces64s_(O), 64s_(E) and cradle members 84_(O), 84_(E) have negligibleimpact on the overall weight of the rotor blade assemblies O_(b), E_(b).Furthermore, relatively minor modifications are required to effect achange in the rotor system configuration, i.e., in the second operatingmode. For example, with respect to the embodiments depicted in FIGS.10a-10c and 13a-13c, the initial radial position of the ball nuts98_(O), 98_(E), or the length of the strap means 116_(O), 116_(E), maybe changed to modify the maximum permissible blade length variation fromodd to even blade assemblies O_(b), E_(b). With respect to the otherdescribed embodiments, the radial distances Rs_(O), Rs_(E) of the stopsurfaces 64s_(O), 64s_(E) may be altered to effect the same result.

Furthermore, the pilot may control blade length, so as to selectivelycommand blade length variations which are less than the maximumpermissible blade length variation. For example, with respect to theembodiments depicted in FIGS. 10a-10c and 13a-13c, the ball nuts 98_(E)or end fixities 114_(E), may be positioned relative to the stop surfaces64s_(O) so as to effect any combination of blade length variation withinthe prescribed ranges. With respect to the other embodiments the ballnuts 98_(O) or end fixities 114_(O) may be similarly positioned toeffect the same result.

Although the invention has been shown and described with respect toexemplary embodiments thereof, it should be understood by those skilledin the art that the foregoing and other changes, omissions and additionsmay be made therein and thereto, without departing from the spirit andscope of the present invention.

What is claimed is:
 1. A Variable Diameter Rotor system (4) forrotorcraft and operative to reduce Blade-Vortex Interaction (BVI) noise,said rotorcraft operating in first and second operating modes,comprising:a rotor hub assembly (6) defining an axis of rotation (8)about which said rotor hub assembIy (6) rotates; odd and even bladeassemblies (O_(b), E_(b)) mounting to and rotating with said rotor hubassembly (6), each of said rotor blade assemblies (O_(b), E_(b)) havinginboard and outboard blade sections (10, 12) defining an internalchamber (64), said outboard blade section (12) telescopically mounted tosaid inboard blade section (10) and being biased outwardly of saidrotational axis (8) by centrifugal forces C_(f) acting thereupon; saidodd and even rotor blade assemblies (O_(b), E_(b)) each defining aradial length R_(O) and R_(E), respectively; means (70) for positioningsaid outboard blade sections (12) with respect to said inboard bladesections (10) such that, in the first operating mode, said radial lengthR_(E) of said even blade assemblies (E_(b)) is equal to said radiallength R_(O) of said odd blade assemblies (O_(b)), and, in the secondoperating mode, said radial length R_(E) is between about 70% to about95% of said radial length R_(O), said positioning means (70) including:acentrifugal restraint assembly (80) disposed in each said internalchamber (64) and disposed in combination with said outboard bladesection (12), said centrifugal restraint assembly (80) being biasedoutwardly of said rotational axis (8) by said centrifugal forces C_(f)acting on said outboard blade section (12); a stop surface (64s) formedinternally of each said internal chamber (64): and actuation means (90),in combination with said centrifugal forces operative fortranspositioning said centrifugal restraint assemblies (80) within saidinternal chambers (64) such that, in the first operating mode, saidcentrifugal restraint assemblies (80) of said even blade assemblies(E_(b)) are disposed in abutting engagement with said stop surfaces(64s_(E)) thereof and said centrifugal restraint assemblies (80) of saidodd blade assemblies (O_(b)) are disposed in abutting engagement withsaid actuation means (90) and, in the second mode, said centrifugalrestraint assemblies (80) are disposed in abutting engagement with saidstop surfaces (64s).
 2. The Variable Diameter Rotor system (4) accordingto claim 1 wherein each said centrifugal restraint assembly (80)includes:a retention block (82) disposed in combination with saidoutboard blade section (12); a cradle member (84) disposed in slidingcombination with an internal wall (64w) of said internal chamber (64);and a connecting means (86) disposed between and mechanically couplingsaid retention block (82) to said cradle member (84); and said cradlemembers (84_(E)) of said even blade assemblies (E_(b)), in the firstoperating mode, being disposed in abutting engagement with said stopsurfaces (64s_(E)) thereof and said cradle members (84_(O)) of said oddblade assemblies (O_(b)) being disposed in abutting engagement with saidactuation means (90) and, in the second operating mode, said cradlemembers (84) are disposed in abutting engagement with said stop surfaces(64s).
 3. The Variable Diameter Rotor system (4) according to claim 2wherein said actuation means (90) includes:a ballscrew assembly (92)disposed in each said internal chamber (64), each said ballscrewassembly (92) including:a threaded ballscrew (94) defining alongitudinal axis (94L) and operative for rotation about saidlongitudinal axis (94L) in a clockwise and counterclockwise direction;and a ball nut (98) disposed in threaded combination with said threadedballscrew (94) and radially outboard of said cradle member (84), saidball nut (98), furthermore, operative for translating linearly alongsaid ballscrew (94) in response to rotation thereof and operative forengaging and disengaging said cradle member (84); and drive means foreffecting rotation of said threaded ballscrews (94) thereby effectinglinear translation of said ball nuts (98).
 4. The Variable DiameterRotor system (4) according to claim 3wherein said ball nuts (98_(O)) ofsaid odd blade assemblies (O_(b)) define a radial distance Rn_(O) fromsaid rotational axis (8), and said ball nuts (98_(E)) of said even bladeassemblies (E_(b)) define a radial distance Rn_(E) from said rotationalaxis (8), said radial distance Rn_(E) being equal to said radialdistance Rn_(O).
 5. The Variable Diameter Rotor system (4) according toclaim 2wherein said cradle member (84) and the tip end of the outboardblade section (12) define a length dimension L therebetween; and whereinsaid stop surfaces (64s_(E)) of said even blade assemblies (E_(b))define a radial distance Rs_(E) from said rotational axis (8), and saidstop surfaces (64s_(O)) of said odd blade assemblies (O_(b)) define aradial distance Rs_(O) from said rotational axis (8), said radialdistance Rs_(E) of said stop surface (64s_(E)) being between about0.7Rs_(O) -0.3L to about 0.95Rs_(O) -0.05L.
 6. The Variable DiameterRotor system (4) according to claim 5 wherein said radial distanceRs_(E) of said stop surface (64s_(E)) is between about 83Rs_(O) -0.17Lto about 0.92Rs_(O) -0.08L.
 7. A Variable Diameter Rotor system (4) forrotorcraft and operative to reduce Blade-Vortex Interaction (BVI) noise,said rotorcraft operating in first and second operating modes,comprising:a rotor hub assembly (6) defining an axis of rotation (8)about which said rotor hub assembly (6) rotates; odd and even bladeassemblies (O_(b), E_(b)) mounting to and rotating with said rotor hubassembly (6), each of said rotor blade assemblies (O_(b), E_(b)) havinginboard and outboard blade sections (10, 12) defining am internalchamber (64), said outward blade section (12) telescopically mounted tosaid inboard blade section (10) and being biased outwardly of saidrotational axis (8) by centrifugal forces C_(f) acting thereupon; saidodd and even rotor blade assemblies (O_(b), E_(b)) each defining aradial length R_(O) and R_(E), respectively; means (70) for positioningsaid outboard blade sections (12) with respect to said inboard bladesections (10) such that, in the first operating mode, said radial lengthR_(E) of said even blade assemblies (E_(b)) is equal to said radiallength R_(O) of said odd blade assemblies (O_(b)), and, in the secondoperating mode, said radial length R_(E) is between about 70% to about95% of said radial length R_(O), said positioning means (70) including:acentrifugal restraint assembly (80) disposed in each said internalchamber (64) and disposed in combination with said outboard bladesection (12), said centrifugal restraint assembly (80) being biasedoutwardly of said rotational axis (8) by said centrifugal forces C_(f)acting on said outboard blade section (12); a stop surface (64s) formedinternally of each said internal chamber (64); and actuation means (90),in combination with said centrifugal forces C_(f), operative fortranspositioning said centrifugal restraint assemblies (80) within saidinternal chambers (64) such that, in the first operating mode, saidcentrifugal restraint assemblies (80) are disposed in abuttingengagement with said stop surfaces (64s) and, in the second operatingmode, said centrifugal restraint assemblies (80) of said even bladeassemblies (E_(b)) are disposed in abutting engagement with saidactuation means (90).
 8. The Variable Diameter Rotor system (4)according to claim 7 wherein each said centrifugal restraint assembly(80) includes:a retention block (82) disposed in combination with saidoutboard blade section (12): a cradle member (84) disposed in slidingcombination with an internal wall (64w) of said internal chamber (64);and a connecting means (86) disposed between and mechanically couplingsaid retention block (82) to said cradle member (84); said cradlemembers (84), in the first operating mode, being disposed in abuttingengagement with said stop surfaces (64s), and said cradle members(84_(E)) of said even blade assemblies (E_(b)), in the second operatingmode, being disposed in abutting engagement with said actuation means(90).
 9. The Variable Diameter Rotor system (4) according to claim 8wherein said actuation means (90) includes:a ballscrew assembly (92)disposed in each said internal chamber (64), each said ballscrewassembly (92) including:a threaded ballscrew (94) defining alongitudinal axis (94L) and operative for rotation about saidlongitudinal axis (94L) in a clockwise and counterclockwise direction;and a ball nut (98) disposed in threaded combination with said threadedballscrew (94) and radially outboard of said cradle member (84), saidball nut (98), furthermore, operative for translating linearly alongsaid ballscrew (94) in response to rotation thereof and operative forengaging and disengaging said cradle member (84); and drive means foreffecting rotation of said threaded ballscrews (94) thereby effectinglinear translation of said ball nuts (98).
 10. The Variable DiameterRotor system (4) according to claim 9wherein said cradle members (84)and the tip end of the outboard blade section (12) define a lengthdimension L therebetween; and wherein said ball nuts (98_(O)) of saidodd blade assemblies (O_(b)) define a radial distance Rn_(O) from saidrotational axis (8), and said ball nuts (98_(E)) of said even bladeassemblies (E_(b)) define a radial distance Rn_(E) from said rotationalaxis (8), said radial distance Rn_(E) being between about 0.7Rn_(O)-0.3L to about 0.95Rn_(O) -0.05L in the second operating mode.
 11. TheVariable Diameter Rotor system (4) according to claim 10 wherein saidradial distance Rn_(E) is between about 0.83Rn_(O) -0.17L to about0.92Rn_(O) -0.08L in the second operating mode.
 12. The VariableDiameter Rotor system (4) according to claim 10 wherein said stopsurfaces (64s_(O)) of said odd blade assemblies (O_(b)) define a radialdistance Rs_(O) from said rotational axis (8), and wherein said stopsurfaces (64s_(E)) of said even blade assemblies (E_(b)) define a radialdistance Rs_(E) from said rotational axis (8), said radial distanceRs_(O) being equal to said radial distance Rs_(E).
 13. A VariableDiameter Rotor system (4) for rotorcraft and operative to reduceBlade-Vortex Interaction (BVI) noise, said rotorcraft operating infirst, second and third operating modes, comprising:a rotor hub assembly(6) defining an axis of rotation (8) about which said rotor hub assembly(6) rotates; odd and even blade assemblies (O_(b), E_(b)) mounting toand rotating with said rotor hub assembly (6), each of said rotor bladeassemblies (O_(b), E_(b)) having inboard and outboard blade sections(10, 12) defining an internal chamber (64), said outboard blade section(12) telescopically mounted to said inboard blade section (10) and beingbiased outwardly of said rotational axis (8) by centrifugal loads C_(f)acting thereupon; said odd and even rotor blade assemblies (O_(b),E_(b)) each defining a radial length R_(O) and R_(E) respectively; means(70) for positioning said outboard blade sections (12) with respect tosaid inboard blade sections (10), said positioning means (70) furtherincluding:a centrifugal restraint assembly (80) disposed in each saidinternal chamber (64), each said centrifugal restraint assembly (80)including:a retention block (82) disposed in combination with saidoutboard blade section (12); a cradle member (84) disposed in slidingcombination with an internal wall (64w) of said internal chamber (64);and a connecting means (86) disposed between and mechanically couplingsaid retention block (82) to said cradle member (84); said cradlemembers (84) being biased outwardly of said rotational axis (8) by saidcentrifugal forces C_(f) acting on said outboard blade section (12);said cradle members (84) and the tip end of said outboard blade sections(12) defining a length dimension L therebetween; a stop surface (64s)formed internally of each said internal chamber (64), said stop surfaces(64s_(O)) of said odd blade assemblies (O_(b)), defining a radialdistance Rs_(O) from said rotational axis (8) and said stop surfaces64s_(E) of said even blade assemblies (E_(b)) defining a radial distanceRs_(E) from said rotational axis (8), said radial distance Rs_(E) ofsaid stop surface (64s_(E)) being between about 0.7Rs_(O) -0.3L to about0.95Rs_(O) -0.05L; and actuation means (90), in combination with saidcentrifugal forces C_(f) acting on said outboard blade section (12),operative for transpositioning said cradle members (84) within saidinternal chambers (64) such that said cradle members (84) are disposedin abutting engagement with said stop surfaces (64s) in one of theoperating modes and in abutting engagement with said actuation means(90) in another of said operating modes; whereby, in the first operatingmode, said actuation means (90) is disengaged from said cradle members(84_(E)) of said even blade assemblies (E_(b)) such that said cradlemembers (84_(E)) thereof are positioned in abutting engagement with saidstop surfaces (64s_(E)) of said even blade assemblies (E_(b)), and saidactuation means (90) is disposed in abutting engagement with said cradlemembers (84_(O)) of said odd blade assemblies (O_(b)) such that said oddand even blade assemblies (O_(b), E_(b)) are fully-extended, and saidradial length R_(E) of said even blade assemblies (E_(b)) is equal tosaid radial length R_(O) of said odd blade assemblies (O_(b)); whereby,in the second operating mode, said actuation means (90) transpositionssaid cradle members (84_(O)) of said odd blade assemblies (O_(b)) suchthat said cradle members (84_(O)) thereof are disposed in abuttingengagement with said stop surfaces (64s_(O)), said odd and even bladeassemblies (O_(b), E_(b)) alternately vary in length, and said radiallength R_(E) is between about 70% to about 95% of said radial lengthR_(O) ; and whereby, in the third operating mode, said actuation means(90) engages said cradle members (84) of said odd and even bladeassemblies (O_(b), E_(b)) for transpositioning said cradle members (84)thereof such that said odd and even blade assemblies (O_(b), E_(b)) arefully-retracted and said radial length R_(E) of said even bladeassemblies (E_(b)) is equal to said radial length R_(O) of said oddblade assemblies (O_(b)).
 14. The Variable Diameter Rotor system (4)according to claim 13 wherein said radial distance Rs_(E) of said stopsurface (64s_(E)) is between about 83Rs_(O) -0.17 L to about 0.92Rs_(O)-0.08L.
 15. The Variable Diameter Rotor system (4) according to claim 13wherein the second operating mode is a descending flight mode.
 16. TheVariable Diameter Rotor system (4) according to claim 13 wherein saidactuation means (90) includes:a ballscrew assembly (92) disposed in eachsaid internal chamber (64), each said ballscrew assembly (92)including:a threaded ballscrew (94) defining a longitudinal axis (94L)and operative for rotation about said longitudinal axis (94L) in aclockwise and counterclockwise direction; and a ball nut (98) disposedin threaded combination with said threaded ballscrew (94) and radiallyoutboard of said cradle member (84), said ball nut (98), furthermore,operative for translating linearly along said ballscrew (94) in responseto rotation thereof and operative for engaging and disengaging saidcradle member (84); and drive means for effecting rotation of saidthreaded ballscrews (94) thereby effecting linear translation of saidball nuts (98).
 17. The Variable Diameter Rotor system (4) according toclaim 16 wherein said ball nuts (98_(b)) of said odd blade assemblies(O_(b)) define a radial distance Rn_(O) from said rotational axis (8),and said ball nuts (98_(E)) of said even blade assemblies (E_(b)) definea radial distance Rn_(E), from said rotational axis (8) said radialdistance Rn_(O), being equal to said radial distance Rn_(E) in thefirst, second and third operating modes.
 18. A Variable Diameter Rotorsystem (4) for rotorcraft and operative to reduce Blade-VortexInteraction (BVI) noise, said rotorcraft operating in first, second andthird operating modes, comprising:a rotor hub assembly (6) defining anaxis of rotation (8) about which said rotor hub assembly (6) rotates;odd and even blade assemblies (O_(b), E_(b)) mounting to and rotatingwith said rotor hub assembly (6), each of said rotor blade assemblies(O_(b), E_(b)) having inboard and outboard blade sections (10, 12)defining an internal chamber (64), said outboard blade section (12)telescopically mounted to said inboard blade section (10) and beingbiased outwardly of said rotational axis (8) by centrifugal forces C_(f)acting thereupon; said odd and even rotor blade assemblies (O_(b),E_(b)) each defining a radial length R_(O) and R_(E), respectively;means (70) for positioning said outboard blade sections (12) withrespect to said inboard blade sections (10), said positioning means (70)further including:a centrifugal restraint assembly (80) disposed in eachsaid internal chamber (64), each said centrifugal restraint assembly(80) including:a retention block (82) disposed in combination with saidoutboard blade section (12); a cradle member (84) disposed in slidingcombination with an internal wall (64w) of said internal chamber (64);and a connecting means (86) disposed between and mechanically couplingsaid retention block (82) to said cradle member (84); said cradlemembers (84) being biased outwardly of said rotational axis (8) by saidcentrifugal forces C_(f) acting on said outboard blade section (12); astop surface (64s) formed internally of each said internal chamber (64),said stop surfaces (64s_(O)) of said odd blade assemblies (O_(b))defining a radial distance Rs_(O) from said rotational axis (8), andsaid stop surfaces (64s_(E)) of said even blade assemblies (E_(b))defining a radial distance Rs_(E) from said rotational axis (8), saidradial distance Rs_(O) being equal to said radial distance Rs_(E) ; andactuation means (90), in combination with said centrifugal forces C_(f)acting on said outboard blade section (12), operative fortranspositioning said cradle members (84) within said internal chambers(64) such that, in one of the operating modes, said cradle members (84)are disposed in abutting engagement with said stop surfaces (64s) and,in a second operating mode, said cradle members are in abuttingengagement with said actuation means (90); whereby, in the firstoperating mode, said actuation means (90) is disengaged from said cradlemembers (84) such that said cradle members (84) are disposed in abuttingengagement with said stop surfaces (64s), said odd and even bladeassemblies (O_(b), E_(b)) are fully-extended, and said radial lengthR_(E) of said even blade assemblies (E_(b)) is equal to said radiallength R_(O) of said odd blade assemblies (O_(b)); whereby, in thesecond operating mode, said actuation means (90) engages said cradlemembers (84_(E)) of said even blade assemblies (E_(b)) fortranspositioning said cradle members (84_(E)) thereof such that said oddand even blade assemblies (O_(b), E_(b)) alternately vary in length andsaid radial length R_(E) is between about 70% to about 95% of saidradial length R_(O) ; and whereby, in the third operating mode, saidactuation means (90) engages said cradle members (84) of said odd andeven blade assemblies (O_(b), E_(b)) for transpositioning said cradlemembers (84) thereof such that said odd and even blade assemblies(O_(b), E_(b)) are fully-retracted and said radial length R_(E) of saideven blade assemblies (E_(b)) is equal to said radial length R_(O) ofsaid odd blade assemblies (O_(b)).
 19. The Variable Diameter Rotorsystem (4) according to claim 18 wherein said actuation means (90), inthe second operating mode, transpositions said cradle members (84_(E))of said even blade assemblies E_(b) such that said radial length R_(E)is between about 83% to about 95% of said radial length R_(O).
 20. TheVariable Diameter Rotor system (4) according to claim 18 wherein thesecond operating mode is a descending flight mode.
 21. The VariableDiameter Rotor system (4) according to claim 18 wherein said actuationmeans (90) includes:a ballscrew assembly (92) disposed in each saidinternal chamber (64), each said ballscrew assembly (92) including: athreaded ballscrew (94) defining a longitudinal axis (94L) and operativefor rotation about said longitudinal axis (94L) in a clockwise andcounterclockwise direction; and a ball nut (98) disposed in threadedcombination with said threaded ballscrew (94) and radially outboard ofsaid cradle member (84), said ball nut (98), furthermore, operative fortranslating linearly along said ballscrew (94) in response to rotationthereof and operative for engaging and disengaging said cradle members(84); and drive means for effecting rotation of said threaded ballscrews(94) thereby effecting linear translation of said ball nuts (98). 22.The Variable Diameter Rotor system (4) according to claim 21wherein saidthreaded ballscrews (94) have threads defining a thread pitch, saidthread pitch of said odd blade assemblies (O_(b)) being greater than andsaid thread pitch of said even blade assemblies (E_(b)) therebyeffecting a pitch differential therebetween; and wherein said cradlemembers (84) and the tip end of the outboard blade section (12) define alength dimension L therebetween; and wherein said ball nuts (98_(O)) ofsaid odd blade assemblies (O_(b)) define a radial distance Rn_(O) fromsaid rotational axis (8), and said ball nuts (98_(E)) of said even bladeassemblies (E_(b)) define a radial distance Rn_(E) from said rotationalaxis (8), said radial distance Rn_(E) being between about 0.7Rn_(O)-0.3L to about 0.95Rn_(O) 0.05L in the second operating mode; andwherein said pitch differential causes said ball nuts (98_(O), 98_(E))of said odd and even blade assemblies (O_(b), E_(b)) to traverse atdifferential linear rates within said internal chambers (64), such thatsaid radial distance Rn_(O) is equal to said radial distance Rn_(E) inthe third operating mode.
 23. The Variable Diameter Rotor system (4)according to claim 21wherein said drive means (100) drives said threadedballscrews (94_(O)) of said odd blade assemblies (O_(b)) at an increasedrotational speed relative to said threaded ballscrews (94_(E)) of saideven blade assemblies (E_(b)), thereby effecting a speed differentialtherebetween; wherein said cradle members (84) and the tip end of theoutboard blade section (12) define a length dimension L therebetween;and wherein said ball nuts (98_(O)) of said odd blade assemblies (O_(b))define a radial distance Rn_(O) from said rotational axis (8), and saidball nuts (98_(E)) of said even blade assemblies (E_(b)) define a radialdistance Rn_(E) from said rotational axis (8), said radial distanceRn_(E) being between about 0.7Rn_(O) -0.3L to about 0.95Rn_(O) 0.05L inthe second operating mode; and wherein said speed differential causessaid ball nuts (98_(O)) of said odd and even blade assemblies (O_(b),E_(b)) to traverse at differential linear rates within said internalchambers (64), such that said radial distance Rn_(O) is equal to saidradial distance Rn_(E) in the third operating mode.